1. Field of the Invention
The present invention relates to a sheet conveying apparatus that conveys sheets and an image forming apparatus provided with the sheet conveying apparatus.
2. Description of the Related Art
In an image forming apparatus, when a skewed sheet feeding occurs, a correct image can not be formed on each sheet. Therefore, conventionally the image forming apparatus has a sheet registration correcting mechanism for correcting a skewed sheet, when skew feeding of the sheet to be conveyed occurs.
In a conventional sheet registration correction, there is a method of correcting the skewed sheet (called a skew correction hereafter) in which two rollers are used to be independently driven, thereby giving a relative speed difference to the rollers. Also, there is a method of correcting the skewed sheet by butting tips of paper sheets, on one side of its vertical surface in a conveying direction (called a skew feed butting correction). Further, generally there are various methods such as obviating the skew feeding by forming the tips of the sheets into a loop state, as illustrated in a registration roller.
As general applications, skew feed butting correction and skew correction are performed to enable print-on-demand image to be formed, from the viewpoint of high precision+high productivity. In addition, from the viewpoint of miniaturization of an apparatus body, the skew correction is advantageous in structure. However, particularly in the skew correction, when a conveyed sheet is shifted in a direction orthogonal to a conveying direction (lateral direction), in order to correct such a shift, sheet lateral registration correction needs to be performed separately. In this case, generally a flow of correcting lateral registration is as follows. Namely, sheet tip registration correction is performed first, and then sheet lateral registration correction is performed.
For example, Japanese Patent Application Laid-Open No. 05-124752 and Japanese Patent Application Laid-Open No. 2000-289889 propose a technique in which a lateral registration of the sheet is detected and according to an amount of detection, the sheet is laterally shifted. According to Japanese Patent Application Laid-Open No. 05-124752, the lateral registration of the sheet is detected in such a manner that a lateral tip of the sheet is detected by a line sensor such as contact image sensor, and a sheet lateral shift amount is detected. Then, the sheet lateral registration is corrected in such a manner that a roller itself conveying the sheet is moved in a direction orthogonal to the conveying direction (=the sheet itself is laterally moved) by an amount according to a detected sheet lateral shift amount.
In addition, as illustrated in Japanese Patent Application Laid-Open No. 10-310288, there is a proposal that by disposing a sensor at a sheet side edge reference position, a sheet side edge is aligned with a sheet side edge reference. This is a technique of correcting the sheet lateral registration by moving the roller itself conveying the sheet, in a direction perpendicularly to the conveying direction.
The documents provide a technique of aligning the sheet with a formed image by laterally moving the sheet itself, as a lateral registration correction part of the sheet. The detection itself includes the detection for deciding a moving amount and the detection for deciding a completion of movement.
However, according to a conventional technique, the lateral registration of the sheet is corrected by laterally shifting the conveying roller. However, this conventional technique involves problems such as described below.
For example, from the viewpoint of a mechanical restriction and productivity, in a sheet lateral registration shift area, the lateral shift of a pair of lateral shift rollers, with a sheet nipped between them, can not occur at a timing of pass of the sheet tips. Therefore, the lateral shift of the sheet lateral registration shift area is performed in many cases, at a timing when the tip of the sheet is conveyed by predetermined amounts after passing through the pair of the lateral shift rollers.
In such a case, for example as illustrated in FIG. 19, when it is assumed that the lateral shift is performed at a position advancing by ¼ of a sheet size A in the conveying direction (broken line in the figure) from the tip of the sheet, no particular problem is involved in the case of a sheet size A. However, in the case of a sheet size B (solid line in the figure) having a greater length than that of the sheet size A in the conveying direction, the lateral shift is performed at the tip side of the sheet. If the length of the size B in the conveying direction is twice the length of the size A, as illustrated in FIG. 19, the following equation is established:(lateral shift roller from tip of sheet):(rear tip of sheet from lateral shift roller)=1:7This reveals that lateral shift rollers 31 perform lateral shift, with sheet nipped between them, at a position extremely closer to the tip portion of the sheet.
Therefore, a lateral shift force is added to only the tip portion of the sheet in a direction X orthogonal to a conveying direction Y, and absolutely no force is added to the rear tip portion of the sheet in the direction X, to set this portion in a free state. Actually, the force added to the sheet in the direction X is unbalanced if the lateral shift is performed with the sheet nipped between the pair of the lateral shift rollers at a biased position as described above, due to a friction and air resistance between the sheet and a conveying path. As a result, accurate lateral shift control is difficult.
Thus, in the case of the structure that “the lateral shift correction is performed always at a predetermined timing from the tip of the sheet irrespective of a sheet size”, the shift force added to the sheet is unbalanced, thus generating a slip, and a stable correction can not be performed. Therefore, there is a problem that the sheet is skewed in some cases.
Note it can be considered that when the lateral shift rollers 31 are disposed at two places of the tip side and the rear tip side in the conveying direction.
And it can be also considered that when the center part of the sheet in the conveying direction is nipped between the lateral shift rollers, a lateral shift operation is started. However, when the lateral shift rollers 31 are disposed at two places of the tip side and the rear tip side in the conveying direction or the lateral shift operation is started with the center part of the sheet nipped by the pair of the lateral shift rollers, a stable correction of a sheet position can not be performed by generating a slip between the lateral shift rollers 31 an the sheet due to a friction between the sheet and the conveying path while the lateral shift rollers 31 is shifting the sheet.